Concentric cylinder assembly for producing pulsed neutrons

ABSTRACT

Pulsed bursts of neutron are produced from a source in which strips of radioactive material are placed on the inner surface of a cylinder, and low Z target material is placed on the outer surface of an inner concentric cylinder. As the inner cylinder is rotated, the radioactive material and the target material are matched to produce a burst of neutrons.

United States Patent [1 1 Coleman et al.

[ Aug. 7, 1973 CONCENTRIC CYLINDER ASSEMBLY FOR PRODUCING PULSEDNEUTRONS [7S] lnventors: Harold L. Coleman, Dayton; Harold A. Malson,Kettering; Howard R. DuFour, New Carlisle; Richard G. Olt, Dayton, allof Ohio [73] Assignee: Monsanto Research Corporation, St.

3,379,884 Youmans 250/83.6 PS

3/1969 Givens 250/83.l l2/l942 Fearon 250/83.6 PS

Primary Examiner-Archie R. Borchelt Assistant Examiner-Davis l. WillisAttorneyL. Bruce Stevens, Jr.

[57] ABSTRACT Pulsed bursts of neutron are produced from a source inwhich strips of radioactive material are placed on the inner surface ofa cylinder, and low Z target material is placed on the outer surface ofan inner concentric cylinder. As the inner cylinder is rotated, theradioactive material and the target material are matched to produce aburst of neutrons.

5 Claims, 4 Drawing Figures l2 l3 /4 /Z CONCENTRIC CYLINDER ASSEMBLY FORPRODUCING PULSED NEUTRONS BACKGROUND OF THE INVENTION The inventionherein described was made in the course of or under a contract with theDepartment of the Navy.

1. Field of the Invention This invention relates to a neutron generatorwherein the neutrons are produced by alpha particles from the decay of aradioactive isotope impinging on a light element. By a criticalarrangement of the elements the emission of neutrons can be stopped orstarted at will.

2. Description of the Prior Art Neutron sources are well-known in theprior art, and have been used for a wide variety of applications. Theyhave been extensively used in oil well logging operations whereinformations are irradiated from a neutron source for the production ofsecondary-radiation. Measurements are made of the secondary radiation toobtain information about the formations under investigation.

For these well logging operations pulsed neutron sources have been foundto be particularly useful, and a number of pulsed radiation sources aretaught by the prior art for these operations. As an example, U.S. Pat.No. 3,435,216 issued Mar. 25, I969 and U.S. Pat. No. 3,389,257 issuedJune 18, I968 teach the use of rotating shutters to pulse the neutrons.

Additionally, U.S. Pat. No. 3,388,253 issued June 11, 1968 rotates atarget past a radioactive source secured to a support for the productionof bursts of primary radiation, and U.S. Pat. No. 2,275,748 issued Mar.10, 1942 suspends a source of alpha particles between the outer edges ofthe two disks which are rotated by means of a motor to produce pulsedneutrons.

Despite these advances made in pulsed neutron sources, all of the priorart sources have certain disadvantages. Because of the penetratingnature of neutrons, it is difficult to stop many of the neutrons using arotating shutter. Additionally, many of these sources, because of theradioisotope used or the configuration of the sources, emit a largeneutron flux when in the off position. Furthermore, the amount ofradioisotope used in the rotating target sources has been found to be alimiting factor to the production of large neutron fluxes.

In addition to these oil well logging operations, neutron sources arewidely used in the start-up of nuclear reactors, and to calibrateinstruments used in the operation of these reactors. Thus, pulsedneutron sources, or on the other hand, neutron sources having apredetermined variable flux, find ready application in this and similarfields.

BRIEF SUMMARY OF THE INVENTION It is therefore one object of the presentinvention to prepare a pulsed or modulated (adjustable flux) neutronsource.

It is another object of the invention to prepare a pulsed neutron sourcehaving a rotating target.

It is yet another object of the invention to prepare a pulsed neutronsource having a greater amount of radioisotope in a small configurationfor more efficient utilization of the target material.

These and other objects of the present invention are achieved by aneutron generator that emits pulsed bursts 0r adjustable fluxes ofneutrons which comprises a hollow cylindrical body; strips ofradioactive material mounted on the inner surface of the hollowcylindrical body, wherein the strips mounted parallel to the axis ofrotation provide a source of radioactive particles; an inner cylinderrotatably mounted inside of the hollow cylindrical body; strips of low Ztarget material affixed to or exposed on the outer surface of the innercylinder, wherein the strips are affixed parallel to the axis ofrotation; and means of rotating the inner cylinder relative to thehollow cylindrical body to bring the strips of target material in closeproximity to the strips of radioactive material to generate theneutrons.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cutaway view of the pulsedneutron source;

FIG. 2 is a cross-sectional view of the pulsed neutron source through 22of FIG. 1;

FIG. 3 is also a cross-sectional view to demonstrate a differentarrangement of the elements to obtain a particular neutron pulse; and

FIG. 4 is an arc of a cross-sectional view, the parts being out ofproportion to show the details of one embodiment.

DETAILED DESCRIPTION Referring now to FIG. 1 a hollow right circularcylinder may be used as the hollow cylindrical body 11. On the interiorsurface of the hollow cylindrical body 11, strips of radioactivematerial 12 are affixed on the surface in a line parallel to the axis ofrotation.

An inner cylinder 13 is rotatable mounted inside the hollow cylindricalbody 11. The inner cylinder 13 may be rotated manually or by means ofmotors or engines, the means not being critical to the presentinvention. Strips of target material 14 are exposed on the outer surfaceof cylinder 13.

The strips of radioactive material 12, in addition to extending alongthe length of the inner cylinder 13 in a plane parallel to the axis ofrotation, are preferably arranged in a configuration to match the stripsof target material 14 on the outer surface of the inner cyliner 13, asshown in FIG. 2. Although the strips of radioactive material need not bematched with the strips of target material, depending upon the needs ofthe user, maximum intensity of neutron pulses are obtained when thestrips are matched. Similarly, the strips could be spiralled on thesurfaces of the respective cylinders, but for ease of fabrication, theyare normally placed in a line parallel to the axis of rotation.

Shaped neutron pulses can be obtained by a critical arrangement of thestrips of radioactive material and the target material, as might occurto one skilled in the art. As an example, a step-wise increase inneutron flux could be obtained by matching three strips of targetmaterial and three strips of radioisotope material within, say, an arcof such as is shown in FIG. 3. Thus, the stepwise increase in neutronflux is achieved when the inner cylinder is rotated to match one stripof target material with one strip of radioactive material, then theinner cylinder is further rotated to match two strips of target materialand radioactive material, etc. Any number of strips could be useddepending upon the desire of the user, and a neutron source of this typecould be used as a variable flux source by matching the appropriatenumber of target strips with radioactive strips.

On the other hand, if only one strip each of radioactive material andtarget material were used, a neutron pulse would be obtained when thetwo strips were matched, but the flux would drop to near zero when thetarget material was rotated to 180 away from the radioactive strip, thusmaking an on-of neutron source for ease of storage when the source isnot in use.

FIG. 4 is an arc of a cross-sectional view of the source to show analternative configuration of the elements. A hollow right cylinder 21having slots 22 through the cylinder wall parallel to the long axis ofrotation is prepared. A shim 23 is fabricated to fit into slot 22, and aradioisotope 24 is deposited on the concave surface of shim 23. Thus,when the shim 23 is placed in the slot 22 and affixed to the cylinder21, a strip of radioactivity parallel to the axis of rotation isprovided. A beryllium inner cylinder 25 is then coated with analpha-stopping material 26, except for a strip 27 parallel to the axisof rotation to expose the beryllium metal. The partially coatedberyllium inner cylinder 25 is then rotatably mounted inside of thehollow cylinder 21.

In operation, the beryllium inner cylinder 25 is rotated in the outercylinder 21 until the strip 27 of exposed beryllium matches the strip ofradioactivity along the inner surface of outer cylinder 21. Theradiation from the radioisotope 24 impinges on the exposed beryllium toproduce neutrons. As the inner cylinder 25 is rotated further so thatthe strips do not match, the alpha-stopping material 26 coating theberyllium prevents the radiation from impinging on the beryllium. Thus,the source may be operated as an on-of neutron source, or bycontinuously rotating the inner cylinder, operate as a pulsed neutronsource.

The radioisotope in the neutron source may be either an alpha, beta orgamma emitting isotope, but alpha emitters have been found to beparticularly useful because the range of the alpha particle in air isshort, and a minimum number of neutrons are produced when the source isin the of position. Polnium-2l0 has been found to be especially usefulin the source as a radioactive material because it is highly radioactiveand is almost a pure alpha emitter, with only negligible beta and gammaradiation. Plutonium-238 or americium-24l may also be used for theradioactive material to overcome the objectionable feature of the shorthalf-life of polonium-210. Other radioisotopes might also be used asmight occur to one skilled in the art.

The target material must contain an element having a relatively lowatomic number (Z) in order to produce neutrons by bombardment fromnuclear radiation. Generally, an element below calcium in the periodictable of the elements must be used, and beryllium and boron have beenfound to be quite useful for this application.

The following examples are given as an illustration of the invention,and not as a limitation to the scope of the invention.

EXAMPLE l About curies of polonium-210 were deposited on the inside of ahollow copper cylinder, approximately four inches long and one inch indiameter. The polonium-2l0 was deposited at l-2 curies per squarecentimeter in two strips parallel to the long axis of the cylinder andcentered 180 apart on the circumference. Be-

ryllium metal was then affixed in two strips 180 apart on the outersurface of an inner copper cylinder, which is concentric to, and fitsinside of, the copper cylinder containing the polonium. The coppercylinder containing the polonium with the inner copper cylinder thereinwas placed in an external stainless steel sheath filled with helium orother inert gas to prevent oxidation of the polonium-210. The innercylinder was then connected to a drive shaft extending through thesheath to the outside of the source to permit rotation of the innercylinder. Thus the inner cylinder may be manually turned to the on" oroff position, or mechanically turned to provide pulsed bursts ofneutrons.

EXAMPLE 2 Two slots were milled 180 apart on the outside of a hollowcopper cylinder which was approximately four inches long and one inch indiameter. Two shims were then fabricated to fill the slots milled on thecopper cylinder. About 10 curies of polonium-210 were then deposited onthe concave surface of each shim, and the shims were then mounted in themilled slots to form two strips of radioactive material, 180 apart. Asolid beryllium cylinder having two strips of masking material 180degrees apart was then coated with nickel to a thickness of one mil,sufficient to stop an alpha particle. When the masking strips wereremoved, two strips of target material 180 apart were exposed. Thecoated beryllium cylinder was then rotatably mounted inside of thecopper cylinder containing the strips of polonium. The two cylinderswere placed in a hermetically sealed stainless steel outer containerfilled with helium. The beryllium cylinder was then rotated using amagnetic coupling through the wall of the stainless steel.

in the on position, or when the beryllium strips and the polonium-210strips were matched, approximately 1.6 X 10 neutrons per second weregenerated. When the inner cylinder was rotated to the off position, only1.6 10 neutrons per second were generated, a significant reduction inneutron flux.

A number of modifications might be made to the above examples. Forinstance the inner cylinder might be rotated by means of a bellowsdeflection coupling for positive drive through a hermetically sealedouter container. Alternatively the hollow cylinder could be rotatedinstead of the inner cylinder to match the strips of radioactivematerial and the target material, or the location of the target materialand the radioactive material might be switched.

Although the invention has been described in terms of specifiedembodiments which are set forth in considerable detail, it should beunderstood that this is by way of illustration only, and that theinvention is not necessarily limited thereto, since alternativeembodiments and operating techniques will become apparent to thoseskilled in the art in view of the disclosure. Ac-

cordingly, modifications are contemplated which can paralled to the axisof rotation to provide a source of radioactive particles;

c. an inner cylinder rotatably mounted inside of the hollow cylindricalbody;

d. at least one strip of low Z target material on the outer surface ofthe inner cylinder, wherein the strip is parallel to the axis ofrotation; and

e. means of rotating the inner cylinder relative to the hollowcylindrical body to bring the strip of target material in closeproximity to the strip of radioactive material to generate the neutrons.

2. A neutron source of claim 1 wherein the radioactive material ispolonium-2l0.

3. A neutron source of claim 1 wherein the low Z target material isberyllium.

4. A neutron source of claim 1 wherein two strips of polonium-2l0 aredeposted approximately 180 apart on the inner surface of the hollowcylindrical body, and two strips of beryllium are mounted approximately180 apart on the outer surface of the inner cylinder.

5. A neutron source of claim 1 which comprises:

a. a hollow cylindrical body rotatable on the long axis thereof andhaving two slots through the cylinder wall which are parallel to theaxis of rotation, the slots being apart;

b. shims mounted on the hollow cylindrical body to fill the slots on theouter surface of the hollow cylindrical body;

c. polonium-ZlO deposited on the concave surface of the shims to supplya strip of radioactive particles to the inside of the hollow cylindricalbody when the shims are in place;

d. a beryllium inner cylinder rotatably mounted in the hollowcylindrical body;

e. a coating of nickel to stop alpha particles, the coating covering thesurface of the beryllium cylinder with two strips approximately 180apart and parallel to the axis of rotation said strips being free ofsaid coating; and

f. means of rotating the inner cylinder relative to the hollowcylindrical body to bring the exposed beryllium strips in closeproximity to the strips of radioactive material to generate theneutrons.

2. A neutron source of claim 1 wherein the radioactive material ispolonium-210.
 3. A neutron source of claim 1 wherein the low Z targetmaterial is beryllium.
 4. A neutron source of claim 1 wherein two stripsof polonium-210 are deposted approximately 180* apart on the innersurface of the hollow cylindrical body, and two strips of beryllium aremounted approximately 180* apart on the outer surface of the innercylinder.
 5. A neutron source of claim 1 which comprises: a. a hollowcylindrical body rotatable on the long axis thereof and having two slotsthrough the cylinder wall which are parallel to the axis of rotation,the slots being 180* apart; b. shims mounted on the hollow cylindricalbody to fill the slots on the outer surface of the hollow cylindricalbody; c. polonium-210 deposited on the concave surface of the shims tosupply a strip of radioactive particles to the inside of the hollowcylindrical body when the shims are in place; d. a beryllium innErcylinder rotatably mounted in the hollow cylindrical body; e. a coatingof nickel to stop alpha particles, the coating covering the surface ofthe beryllium cylinder with two strips approximately 180* apart andparallel to the axis of rotation said strips being free of said coating;and f. means of rotating the inner cylinder relative to the hollowcylindrical body to bring the exposed beryllium strips in closeproximity to the strips of radioactive material to generate theneutrons.